Generally, a wind turbine is a rotating machine that converts the kinetic energy of wind into mechanical energy and, when used for power generation, converts this mechanical energy into electrical energy. Due to their ability to generate electrical power without consuming fossil fuels, wind turbines are increasingly being used as an alternative energy source for providing power to the electrical grid. A wind turbine used for electrical power generation typically includes a rotor having a plurality of blades configured to capture wind power. The rotor is coupled to a generator that converts the rotational energy of the rotor into electrical energy. As the wind speed increases above the minimum or “cut-in” speed (WMIN) for the turbine, the rotor begins to rotate so that the wind turbine can begin producing electrical power. The power output of the wind turbine generally increases with wind speed until the wind speed reaches a nominal or rated wind speed (WR) for the wind turbine. Above the rated wind speed WR, the power output of the wind turbine is limited by the rated power output (PR) of the generator. As the wind speed increases further, the wind speed may reach a cut-out or furling speed (WC), at which point the wind turbine may be shut down to prevent damage to the rotor and/or generator.
Because the output of the generator varies with wind conditions, the generator is normally coupled to the grid by a power converter that conditions the power output of the generator to meet grid voltage, current, phase, and frequency demands. To provide utility scale power to the electrical grid, a wind power system may include one or more wind turbines, with a typical wind power system comprising a wind farm having multiple wind turbines ganged together to provide power to the grid at a common connection point. The collective output of the wind farm at the common connection point may be controlled by a centralized power plant controller (which may be part of a Supervisory Control and Data Acquisition (SCADA) system) that interacts with individual wind turbine controllers to meet electrical grid demands.
Each wind turbine may be controlled—either remotely or locally—by a controller that controls the rotor speed and power output of the wind turbine. The turbine controller thereby keeps the wind turbine operating within its design parameters as well as according to grid power demands. To this end, the turbine controller may include a pitch controller that adjusts the amount of wind power captured by the rotor by adjusting the pitch of the rotor blades. The turbine controller may also include a power converter controller that adjusts the electrical power provided to the grid by the power converter. The amount of wind power captured by the rotor and the amount of electrical power provided to the grid may thereby be selectively adjusted by the turbine controller to control both the speed of the rotor and the electrical power output of the generator.
The amount of power associated with wind is proportional to the cube of the wind's velocity. Thus, a rotor can produce more mechanical power from wind having a relatively high velocity than from wind having relatively low velocity. The efficiency with which the rotor converts the energy contained in the wind into mechanical energy is known as the power coefficient (CP) of the rotor, and is theoretically limited to a maximum of 59% by Betz' Law. The actual power coefficient for a rotor will typically be less than the Betz limit, and depends on the design and operating parameters of the rotor. The amount of wind power that is captured and converted into mechanical power by the rotor, or the “aerodynamic power” is thus a product of the power coefficient of the rotor and the power available from the wind. For a given wind speed, the rotor will have a maximum, or optimal power coefficient CP that is achieved at a particular blade pitch setting and rotational speed. This optimal power coefficient thus sets an upper limit on how much power the turbine can generate at that wind speed. At wind speeds below WR, conventional wind turbines are thus operated with blade pitch and rotational speed settings that optimize CP to maximize power capture. However, as the wind speed increases above WR, aerodynamic power is limited by adjusting the blade pitch to reduce CP. In conventional wind power systems at wind speeds above WR, the rotor is controlled by adjusting the blade pitch in response to changes in wind speed so that captured aerodynamic power does not exceed the rated power of the generator. That is, when the available aerodynamic power is greater than or equal to the rated output of the wind turbine, blade pitch becomes the primary control means used to maintain turbine output at a constant level.
Wind turbines are normally operated to produce the maximum amount of electrical power possible under existing wind conditions so that other sources of electrical power on the grid may be throttled back to conserve non-renewable energy sources. Referring now to FIG. 1, a graphical diagram 10 illustrates an exemplary power curve 12 showing this maximum or “available power” as a function of wind speed. In a low-wind region 14 of the available power output curve 12 between the cut-in wind speed WMIN and the rated wind speed WR, the available aerodynamic power is less than the rated power output PR of the wind turbine generator. Thus, the available power 12 in the low wind region 14 is limited by the wind power that the rotor can capture while operating at optimal blade pitch and rotational speed. In a high-wind region 16 between the rated wind speed WR and the cut-out wind speed WC, the available aerodynamic power is greater than the rated output power PR of the wind turbine generator. The available power 12 in the high-wind region 16 is thus limited to PR.
To maximize wind turbine power output, conventional turbine controllers are configured to output the available power 12 by operating in one of two control modes depending on wind speed: (1) a partial-load control mode that operates in the low-wind speed region 14, or (2) a full-load control mode that operates in the high-wind speed region 16. The turbine controller thus switches from partial-load to full-load control mode in a transition region 17 of the available power curve 12. The turbine controller thereby controls the power output of the wind turbine so that the turbine is producing power at the available output power level 12 for the current wind speed. When operating in the partial-load region, the controller adjusts the pitch and speed of the rotor to optimize CP so that the wind turbine captures as much wind energy as possible. That is, the blade pitch and rotor speed are adjusted to optimize CP for the current wind speed. In the partial-load region, the blade pitch is maintained at an optimum wind capture angle that does not typically change rapidly or frequently with wind speed. The rotor speed may then be set to optimize wind energy capture for the current wind speed by adjusting the amount of power being provided to the grid by the power converter until an optimal tip speed ratio is achieved. In contrast, when operating in the full-load region, the turbine controller adjusts the pitch of the rotor blades so that the rotor only captures enough of the available wind energy to operate the generator at its rated output. Thus, in the full-load region, the electrical power output is maintained at a relatively constant value by the power converter controller, and the blade pitch is adjusted by the wind turbine controller in response to changes in wind speed to maintain the rotor at a generally constant speed and power output level.
A conventional wind turbine controller operating in a power-optimal control mode typically operates as a full-load controller when the wind speed exceeds WR, and as a partial-load controller when the wind speed is below WR. In the full-load control mode of operation, the power converter control system is provided with a fixed power reference signal by the power plant controller, and the rotor speed is controlled by the pitch controller to maintain constant power output. To this end, the pitch controller pitches the blades out of the wind to reduce the power coefficient of the rotor in response to increases in wind speed, and pitches the blades into the wind to increase the power coefficient in response to reduced wind speed. In the partial-load control region that operates at wind speeds below WR, the pitch controller adjusts the blade pitch position to one or more pre-defined optimal positions (e.g., 0°) that provide optimal wind energy capture for the current wind speed. The power converter controller is then provided with a power set-point that couples sufficient electrical power to the grid to maintain the rotational speed of the rotor at an optimal level for the capturing wind energy at the current wind speed. Pitch control thus provides the primary means of controlling output power when the controller is operating in the full-load region of the power output curve 12, and the power converter provides the primary means of controlling output power when the wind turbine controller is operating in the partial-load region of the power output curve 12. Because the pitch control system—rather than the power converter control system—is primarily responsible for compensating for the stochastic behavior of the wind when the controller is operating in the full-load control region, pitching activity is typically significantly higher when the controller is operating in full-load control mode than in partial-load control mode.
At times, the plant controller may request a specific power production level from the turbine which is lower than the available power level. This is commonly referred to as de-rating, and may be used during times of reduced grid demand or to provide an operating reserve to improve grid stability. De-rating is accomplished in conventional systems by providing a power control signal to the turbine controller that causes the controller to reduce maximum turbine output power below the rated output power PR. The maximum power output of the turbine is thereby limited to the de-rated power PD. An exemplary de-rated power output curve 18 thus limits power to PD at wind speeds above WD, which is the wind speed at which the rotor can produce aerodynamic power equal to the de-rated power level. As a result of this lower power demand, the transition region 17 shifts as indicated by arrow 19 so that the turbine controller switches between partial-load and full-load operation at WD rather than WR. Thus, if the turbine is operating at a wind speed between WD and WR under de-rated operating conditions, the wind turbine controller will operate in the full-load control region. This is in contrast to turbine controller operation when the power plant controller is implementing a power-optimal solution, which would implement partial-load turbine control at wind speeds between WD and WR. Therefore, as a consequence of de-rating, the range of full-load operation is extended to wind speeds between WD and WR. Blade pitching activity may thereby be increased over a greater operating range of wind speeds in de-rated wind turbines as compared wind turbines being operated at full capacity. The higher the de-rating (i.e., the lower the power reference signal provided to the wind turbine controller), the further full-load operation is extended below the rated wind speed WR, and the more blade pitching activity is increased. Conventionally controlled wind turbine systems may therefore experience greater pitch system wear under de-rated operation than during full power operation.
Thus, there is a need for improved systems, methods, and computer program products for controlling wind power systems under de-rated conditions that reduces wear to pitch control systems.